Ar-matrix IR spectra of 5-halouracils interpreted by means of DFT calculations

The infrared low-temperature Ar-matrix spectra of 5-halouracils and unsubstituted uracil were measured and interpreted in terms of the spectra calculated at the DFT/B3PW91/6-311G** level followed by a potential energy distribution (PED) analysis. For the PED analysis, the sets of halouracil mode definitions were constructed so that dissimilarities in the interpretations of the different spectra were minimized. Anharmonic frequency calculations enabled more light to be shed on the Fermi resonance (FR) phenomena occurring in the v(C=O) stretching vibrations region. For each halouracil vibrational spectrum, several FRs manifest themselves in the v(C=O) stretching vibrations region. We show that the most frequent components participating in these resonances are the v(C-4=O-10) frequency, a beta(N-H) mode frequency, and a beta(C=O) mode frequency. The experimental v(N-H) frequencies are reproduced by the calculated anharmonic frequencies better than by the scaled harmonic ones, and the v(C=O) frequencies respond in the opposite manner. The experimental frequencies located below 1500 cm(-1) are reproduced equally well by the two kinds of calculations.